This invention relates to an optical system for synthesizing a plurality of light beams, which is used in an optical information recording/reproducing apparatus designed to record optically image information and the like onto an optical disc and reproduce the same.
Generally known as this type of optical system is the one which uses a dichroic mirror. The optical system 100 is constructed as shown in FIG. 1. Referring to FIG. 1, there are shown laser beam sources 102 and 104, collimate lenses 106 and 108, dichroic mirrors 110 and 112, relay lenses 114, 116 and 118, a beam splitter 120, a .lambda./4 plate 122, an objective 124, an optical disc 126, and photo detectors 128 and 130. The light beam of wavelength .lambda..sub.1 emitted from the laser beam source 102 is allowed to pass through the dichroic mirror 110. The light beam of wavelength .lambda..sub.2 emitted from the laser beam source 104 is reflected by the dichroic mirror 110 and is thus synthesized with the light beam emitted from the laser beam source 102. These light beams are introduced into the objective 124 through the relay lens 114, the .lambda./4 plate 122 and are thus irradiated onto prescribed points on the optical disc 126.
In this way, the dichroic mirror 110 permits the passage therethrough of the light beam of wavelength .lambda..sub.1 and at the same time causes the reflection of the light beam of wavelength .lambda..sub.2. However, the following problems are raised where the light beams are synthesized by the dichroic mirror. That is, on account of the characteristic of the dichroic mirror, it is necessary that the wavelengths .lambda..sub.1 and .lambda..sub.2 of the light beams differ from each other to an extent of 100 nm. The gas laser beams such as, for example, Ar laser, He-Ne laser or the like can satisfy this requirement to the full extent. The semi-conductor laser of GaAlAs, which has recently been widely used, has the inconvenience that it necessitates the use of the lights at both ends of the wavelength zone wherein the light beams are stably obtained. Further, where the light beams of different wavelengths are used, it is necessary to take measures for removing the chromatic aberration from the objective 124, relay lenses 114, 118, etc. As a result, the optical system becomes complicated in construction and great in weight.